It is well known in the art to construct solenoids in which the armature assembly is made of a primary plunger and one or more secondary plungers that are serially arranged along the path which the magnetic flux will take when the solenoid is energized. More specifically, it is well known to provide an electromagnet of the type last stated in which the plurality of series air gaps in the magnetic circuit are progressively closed when the solenoid is energized, all armatures of the series being mechanically interconnected which compel the main armature to move towards the zone of maximum tractive effort as the armature situated in such zone moves towards the core or anvil, or towards a supplemental armature which has been attracted to the anvil. The U.S. Pat. No. 1,817,592 is directed to such a solenoid.